This invention relates to digital data detection systems, and more particularly to an optical data detection system that detects data marks representing data transitions.
In an optical storage system, data is recorded on optical media by placing marks on the media, typically in concentric or spiral tracks, that represent the data to be stored. These marks alter the reflectivity or transmissivity of the media at the point where the mark is located. Stored data is subsequently read by directing a narrowly focused beam of light to the data track where the data is recorded and by monitoring this beam as it reflects off of, or passes through, the media. The intensity (or other characteristic, such as phase where coherent light is used) of the reflected or transmitted beam is modulated in accordance with the data patterns marked on the media. Hence, by monitoring the intensity (or other characteristic) of the reflected or transmitted beam, the data stored on the media can be detected.
Generally, digital data is represented on optical media by having a low reflectivity or transmissivity mark represent one digital state, and a high reflectivity or transmissivity mark represent the other digital state. A long string of all ones or zeros marked as one continuous state on the media, where the media normally exhibits the other continuous state (e.g., a long strip of high reflectivity placed on a normally low reflectivity media surface), is generally undesirable because continuous energy in some form, such as a laser beam, must be directed to the surface of the media--typically a rotating disk--at substantially the same power levels. Maintaining the same power levels over relatively long time periods is difficult to achieve. Further, electrical or optical noise can significantly alter what would otherwise be a continuous power level. Moreover, a sufficient number of data transitions, where data transitions are used to define the boundaries between data bits, is needed to generate the synchronous clock signals used to recover or detect the data. Hence, digital coding is generally used, such as a 2,7 code (well known in the art), prior to marking the data on the media in order to preclude the possibility of a long string of all zeros or ones from occurring. (A 2,7 code ensures that no fewer than 3 encoded bits nor no more than 8 encoded bits of data will occur without a data transition.) However, even when a 2,7 code (or other suitable code) is used, constant power levels must still be maintained for time periods substantially longer than one bit time. When semiconductor (diode) lasers are used as the source of the marking energy, or when the source of marking energy is being regularly switched on and off, maintaining constant power levels for even a few bit periods may be difficult. This is because turn-on transients, temperature effects, etc., all influence the initial power levels as the device is first turned on. Accordingly, there is a need in the art for an optical data marking scheme wherein the marking energy need only be on for very short time periods. Such short time periods would advantageously further reduce the adverse contribution of electrical and optical noise to the marking process.
It is known in magnetic recording art to represent digital data by changes in magnetic flux that occur at the point of data transitions in the data to be recorded. When this technique is used, and when the magnetic flux changes are subsequently detected by a magnetic read head, a series of pulses are generated that represent the data transitions that have been detected. The informational content of the data is then found in the spacing or distance between adjacent pulses. In accordance with the invention disclosed herein, such a data-transition marking scheme could also be used to optically record digital data. That is, a single pulse or spot could be optically marked on the media to represent a data transition. A suitable code, such as 2,7 code, could still be used to ensure that a sufficient number of data transitions occurred. The informational content of the data would then be found in the spacing or distance between adjacent spots.
If such a "pulsed optical" data marking scheme is employed, and if the spots are subsequently detected using conventional optical detection techniques, the distance between adjacent spots can only be accurately determined if the spots are of uniform size. This, in turn, requires that the write power of the marking laser beam (or other energy source) be maintained substantially constant. As indicated previously, this is not an easy task, especially when the source of energy is being pulsed on and off. What is needed is a detection system that can accurately measure the spacing or distance between adjacent spots even though the spots may be of non-uniform size. In such a case, one need not be concerned with maintaining the energy level of the writing source, e.g. the write laser beam, at constant levels. Moreover, if spot size were not important, then intensity variations caused by transmissivity or reflectivity changes of the media (which may occur over time) would likewise be of little consequence. The present invention is directed to a detection system wherein spot size may vary without introducing significant errors in the detection process of determining the spacing between adjacent spots.
Optical data detection schemes known in the art, especially where relatively long sequences of one reflectivity or transmissivity state occur, may also disadvantageously produce a dc bias or offset into the detected signal. This offset must be removed in order to maintain the integrity of the detected signal, which removal, if possible at all, may significantly complicate the circuitry used to perform the detection function. A pulsed optical data detection system, as disclosed herein, would advantageously eliminate many of the concerns associated with dc offset because the detection scheme is pulse position sensitive and its matched filtering action effectively removes dc and low frequency components.